Remove outdated strip-js dependency

[dprothero]

The strip-js dependency was used to sanitize the redirect_to query parameter to thwart an XSS attack. It worked, but it is no longer maintained, and it brought along a number of additional dependencies, some of which have documented security vulnerabilities.

The end-goal is to just ensure a valid URL is provided here, so we pass the string to the URL() constructor which will verify it's a valid URL as well as escape anything that might otherwise allow an XSS attack.

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